The phase behavior as well as the thermal and structural behavior of many lipid systems such as chocolate, butter, margarine, milk fat and its fractions have been thoroughly studied by traditional X-ray diffraction techniques. The effect of the addition of emulsifiers or additives on crystallization kinetics of fat systems has also been investigated by X-ray. More recently, the polymorphic behavior of a new fat, cupuassu fat, with properties very similar to cocoa butter, has also been described. This behavior is very relevant since it is related to functional properties of final products. Studies with small angle X-ray scattering (SAXS) allowed describing the structural dynamics of several fats such as milk fat fractions in the early stage of crystallization. This early stage of crystallization is very important since it determines the later evolution of the system. A synchrotron source allows diffraction patterns to be acquired during real-time crystallization, and therefore, further and less speculative information about mechanisms of action can be obtained. SAXS was a valuable tool to explain some unexpected behavior in milk fat isothermal crystallization and also to clarify the effect of sucrose ester on fat crystallization. Several reports have dealt with the latter subject, both in bulk and in emulsion systems, but some of these results may be considered contradictory. With the aid of SAXS, it was possible to improve our understanding of the mechanism of action of a palmitic sucrose ester on low trans fat blends since it was strongly related to the effects on fat polymorphism. © 2012 by Nova Science Publishers, Inc. All Rights Reserved.